The BEOL memories such as RRAM (Resistive Random Access Memory), PRAM (Phase Change Random Access Memory), and MRAM have a resistive device as a memory element. Because high speed access and non-volatility at power off are promised by these devices, they may replace existing memories and create new markets.
FIG. 1B illustrates a typical in-process MTJ memory device 19 at a selected stage during the fabrication process where redeposition material 110 is coating the patterned pillar. The memory device 19, which includes a top electrode 17, a bottom electrode 13 and the magnetic tunnel junction (MTJ) memory element between the electrodes, is fabricated in a pillar shape using conventional lithography and dry etching process. The MTJ memory element typically includes a pinned magnetic layer and a free magnetic layer with a barrier layer 15 in between. In the figure the terms upper magnetic layer 16 and lower magnetic layer 14 are used because either one can be the pinned layer and the other one will be the free layer. The lower magnetic layer 14 is deposited first and will also be called the first magnetic layer. The MTJ layers and bottom electrode are etched conventionally with ion milling or high biased reactive ion etching where elements are mainly removed mechanically. Mechanically etched material is easily redeposited and the redeposited metal is electrically conductive. The redeposition layer 110 forms on MTJ sidewall especially on the barrier layer. The pinned layer and the free layer can be electrically shorted by the redeposition layer 110, which makes the device inoperable.
The source of the redeposition material can be the MTJ stack itself or previously deposited metal layers under the MTJ stack. Once the etching process that forms the pillars passes through the barrier layer, the exposed sidewall of the barrier layer becomes susceptible to being shorted with the redeposited metal. After the bottom electrode layer is etched away, previously deposited metal in the MTJ landing pad and peripheral circuits is exposed to the etching ambient and can be sputtered out and redeposited on the sidewall.